The kernel random-number generator relies on entropy gathered from
device drivers and other sources of environmental noise to seed a
cryptographically secure pseudorandom number generator (CSPRNG). It
is designed for security, rather than speed.
The following interfaces provide access to output from the kernel
CSPRNG:
* The /dev/urandom and /dev/random devices, both described in
random(4). These devices have been present on Linux since early
times, and are also available on many other systems.
* The Linux-specific getrandom(2) system call, available since Linux
3.17. This system call provides access either to the same source
as /dev/urandom (called the urandom source in this page) or to the
same source as /dev/random (called the random source in this
page). The default is the urandom source; the random source is
selected by specifying the GRND_RANDOM flag to the system call.
(The getentropy(3) function provides a slightly more portable
interface on top of getrandom(2).)
Initialization of the entropy pool
The kernel collects bits of entropy from the environment. When a
sufficient number of random bits has been collected, the entropy pool
is considered to be initialized.
Choice of random source
Unless you are doing long-term key generation (and most likely not
even then), you probably shouldn't be reading from the /dev/random
device or employing getrandom(2) with the GRND_RANDOM flag. Instead,
either read from the /dev/urandom device or employ getrandom(2)
without the GRND_RANDOM flag. The cryptographic algorithms used for
the urandom source are quite conservative, and so should be
sufficient for all purposes.
The disadvantage of GRND_RANDOM and reads from /dev/random is that
the operation can block for an indefinite period of time.
Furthermore, dealing with the partially fulfilled requests that can
occur when using GRND_RANDOM or when reading from /dev/random
increases code complexity.
Monte Carlo and other probabilistic sampling applications
Using these interfaces to provide large quantities of data for Monte
Carlo simulations or other programs/algorithms which are doing
probabilistic sampling will be slow. Furthermore, it is unnecessary,
because such applications do not need cryptographically secure random
numbers. Instead, use the interfaces described in this page to
obtain a small amount of data to seed a user-space pseudorandom
number generator for use by such applications.
Comparison between getrandom, /dev/urandom, and /dev/random
The following table summarizes the behavior of the various interfaces
that can be used to obtain randomness. GRND_NONBLOCK is a flag that
can be used to control the blocking behavior of getrandom(2). The
final column of the table considers the case that can occur in early
boot time when the entropy pool is not yet initialized.
┌──────────────┬──────────────┬────────────────┬────────────────────┐
│Interface │ Pool │ Blocking │ Behavior when pool │
│ │ │ behavior │ is not yet ready │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│/dev/random │ Blocking │ If entropy too │ Blocks until │
│ │ pool │ low, blocks │ enough entropy │
│ │ │ until there is │ gathered │
│ │ │ enough entropy │ │
│ │ │ again │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│/dev/urandom │ CSPRNG │ Never blocks │ Returns output │
│ │ output │ │ from uninitialized │
│ │ │ │ CSPRNG (may be low │
│ │ │ │ entropy and │
│ │ │ │ unsuitable for │
│ │ │ │ cryptography) │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ Does not block │ Blocks until pool │
│ │ /dev/urandom │ once is pool │ ready │
│ │ │ ready │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ If entropy too │ Blocks until pool │
│GRND_RANDOM │ /dev/random │ low, blocks │ ready │
│ │ │ until there is │ │
│ │ │ enough entropy │ │
│ │ │ again │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ Does not block │ EAGAIN │
│GRND_NONBLOCK │ /dev/urandom │ once is pool │ │
│ │ │ ready │ │
├──────────────┼──────────────┼────────────────┼────────────────────┤
│getrandom() │ Same as │ EAGAIN if not │ EAGAIN │
│GRND_RANDOM + │ /dev/random │ enough entropy │ │
│GRND_NONBLOCK │ │ available │ │
└──────────────┴──────────────┴────────────────┴────────────────────┘
Generating cryptographic keys
The amount of seed material required to generate a cryptographic key
equals the effective key size of the key. For example, a 3072-bit
RSA or Diffie-Hellman private key has an effective key size of 128
bits (it requires about 2^128 operations to break) so a key generator
needs only 128 bits (16 bytes) of seed material from /dev/random.
While some safety margin above that minimum is reasonable, as a guard
against flaws in the CSPRNG algorithm, no cryptographic primitive
available today can hope to promise more than 256 bits of security,
so if any program reads more than 256 bits (32 bytes) from the kernel
random pool per invocation, or per reasonable reseed interval (not
less than one minute), that should be taken as a sign that its
cryptography is not skillfully implemented.

This page is part of release 5.01 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-03-13 RANDOM(7)